In a 5-4 decision, the court called into question the constitutionality of all federal and state regulation of independent corporate political advocacy, including a federal law dating back to 1947 and the laws of dozens of states.

President Obama today assailed the ruling and promised a "forceful response" to the court's decision, which he says gives "a green light to a new stampede of special interest money in our politics."

Writing for the majority, Justice Anthony Kennedy concluded, "When Government seeks to use its full power, including the criminal law, to command where a person may get his or her information or what distrusted source he or she may not hear, it uses censorship to control thought. This is unlawful."

Lawyers for the Obama administration had argued that such a ruling would "make vast sums of corporate money available for overt electioneering."

While corporations and unions may now spend more freely, the ruling does not overturn an existing prohibition on direct contributions to candidates.

At issue were two previously decided cases -- Austin v. Michigan State Chamber of Commerce and McConnell v. Federal Election Commission -- which upheld restrictions on corporations and non-profits, making it a felony to advocate for or against a candidate weeks before an election.

The court today overturned portions of that precedent, writing, "The First Amendment does not permit laws that force speakers to retain a campaign finance attorney, conduct demographic marketing research or seek declaratory rulings before discussing the most salient political issues of our day."

The court rejected a narrower ruling suggested by the government by saying, "The court cannot resolve this case on a narrower ground without chilling political speech."

Justice John Paul Stevens, in a dissenting opinion joined by Justices Ruth Bader Ginsburg, Stephen Breyer and Sonia Sotomayor, said the court's decision "operates with a sledge hammer rather than a scalpel" and warned that it may "undermine the integrity of elected institutions across the nation.

"The court's opinion is thus a rejection of the common sense of the American people," Stevens wrote, "who have recognized a need to prevent corporations from undermining self-government since the founding and who have fought against the distinctive corrupting potential of corporate electioneering since the days of Theodore Roosevelt."

McCain-Feingold Campaign Finance Overhaul Gutted?

Today's ruling is a setback for generations of reformers who have sought to limit corporate and union power in U.S. elections.

Republican Sen. John McCain of Arizona and Democratic Sen. Russ Feingold of Wisconsin co-sponsored legislation in 2002 that became law, setting set strict limitations on independent corporate and union expenditures in elections.

It bans corporations and unions from funding advertisements 30 days before primary elections and 60 days before general elections. The law also bans ads that expressly advocate the election or defeat of individual candidates.